First off, there are usually multiple reasons for most outcomes. I do not think that marketing alone killed the Turbografx-16: there was Nintendo's monopolistic tactics that were later ruled illegal, the resulting lack of 3rd party support, the idiotic one controller only port on the system necessitating a multitap to play 2 players, and many good Japanese games never made their way across the ocean. However, I do think that a strong case can be made that the image projected by the marketing of the Turbografx-16 was severely uncool enough to dissuade prospective buyers.
Exhibit A:
Why? Who ever thought kids would find this "hero" appealing, much less move units? A sideways hat and sunglasses can only do so much... I cringed when I first saw this. I knew Sega and Nintendo had the TG16 on the ropes, but I hoped it would put up a fight until the end. Johnny Turbo was the white towel.
Exhibit B:
Perhaps if the entire video game playing demographic of the '90s were 2nd to 3rd graders, this would have been a slam dunk. The image by itself is fine. Better than fine, actually. Zonk is cool. The text is a disaster, however. Sticking the tongue out while performing surfing moves out of context is never cool, regardless of how many inches someone happens to be. This, sir, is reverse lemonade making.
Exhibit C:
Uhhh... I guess there was... a... hmm... Just look at it!
What the hell, man!! Seriously! Only Johnny Turbo could make a Star Wars allusion lame. Perhaps 95% of the TG-16's lameness in the advertising is Johnny Turbos fault, and maybe Johnny T came along so late on in the 16-bit war that he was merely the wedgie-ready final nail in the already rotting corpse's coffin, but gadzooks! Any cache of cool the Turbografx might have been holding onto withered and/or died once it caught wind of Johnny boy.
End rant.